The present invention relates to a hydrocracking process. More particularly, the invention relates to a hydrocracking process which yields diesel fuels with increased cetane levels.
Due to upcoming global environmental and governmental mandates, petroleum refiners are seeking the most cost-effective means of improving the quality of their diesel fuel products. The new European Union (EU) diesel cetane number specification of 58 in the year 2005 will require existing processes to be upgraded or the development of new processes.
Aromatic saturation has been commonly utilized to upgrade the cetane level of diesel fuels. However, even with complete aromatic saturation, the cetane level of diesel fuels is only marginally improved; especially those fuels derived from thermal cracking processes such as light cycle oil and coker gas oil. This limited improvement in cetane levels is due to the fact that aromatic saturation can only make low cetane naphthenic species, not the high cetane components such as normal paraffins and iso-paraffins.
A process that increases diesel cetanes through selective ring-opening of naphthenic species, while avoiding cracking the beneficial diesel fuel range paraffins to naphtha and gaseous by-products is therefore desirable. Prior attempts to further increase product cetane levels through selective ring opening of the hydrogenated naphthenic intermediates have not been very successful for a number of reasons.
First, the conventional hydrocracking catalysts are not very selective and cannot be limited to opening naphthene rings, without concurrently cracking some of the paraffinic components. Thus, they frequently result in high diesel yield loss and high yield of gaseous by-product.
Secondly, commercial hydrocracking catalysts which rely on acidity as the active ring opening site will also catalyze increased branching of the resulting naphthenes and paraffins. This branching or isomerization results in cetane loss. Consequently, the more hydroisomerization a given catalyst exhibits, the more cetane loss the diesel products suffer. Typically, as a result of hydroisomerization activity, a cumulative loss of 18-20 cetane numbers is observed for each methyl branching increase.
Thirdly, regardless of the cracking mechanism, molecular weight reduction results in cetane loss when similar molecular structure types are preserved. Normally, a decrease of 3-4 cetane numbers per carbon loss is observed. Thus, endpoint cracking frequently results in cetane loss.
In light of the disadvantages of the conventional processes, there remains a need for a hydrocracking process that produces an increased cetane number without the corresponding diesel yield loss.